1. Field of the Invention
The present invention relates to an ink-jet print head having improved adhesion with time, to its process of manufacturing and to its use in combination with a water-based ink containing acidic species.
2. Description of Related Art
The ink-jet print head of an ink-jet printer generally comprises a substrate, a barrier layer and a nozzle plate. The substrate is generally made of silicon. Various layers are deposited on a face of the silicon substrate to make up the ejection resistors and the active electronic components. The barrier layer is generally made of a photopolymer. Using photolithographic techniques, the ejection chambers and the micro-hydraulic conducts for the ink delivery are realised in the photopolymer barrier layer. The nozzle plate is generally made of a metal, such as gold plated nickel. The nozzle plate provided with ejection nozzles made in correspondence with the ejection resistors and the ejection chambers is attached to the barrier layer. This kind of print heads are usually called “hybrid print heads”.
In recent years, the nozzle plate has been made integrally with the barrier layer. The layer forming the barrier layer and the nozzle plate is known in the art as structural layer. In such a case, the manufacturing process includes a step of forming a pattern of the ejection chambers and the micro-hydraulic conducts with a soluble resin, a step of coating a photopolymer covering the soluble resin pattern, a step of forming orifices in the photopolymer in correspondence of the ejection chambers over the ejection resistors, a step of curing the photopolymer, and a step of dissolving the soluble resin. This kind of print heads are usually called “monolithic print heads”.
There are some problems that arise with respect to the photopolymer employed to make either the barrier layer or the structural layer.
A first problem is that the ink chemically attacks the photopolymer material and causes either leakage between the channels and/or leakage to the outside of the print heads and also causes swelling of the barriers. Swelling results in a change in channel geometry and a degradation from optimised performance.
The main problem, however, is adhesion of the photopolymer layer to the substrate and/or to the nozzle plate. Conventional ink-jet print heads comprise nozzle plates having a surface of gold or other noble metals and also, the ejection resistors and the active electronic components realised on the substrate often comprise metal surfaces of gold or other materials showing low adhesion characteristics; therefore, the problem of adhesion is linked with the chemical nature of the surface.
Further, the adhesion of the photopolymer layer to the substrate and/or to the nozzle plate is also jeopardised by the mechanical strength of the photopolymer material, in particular when the manufacturing process of the print head requires thermal treatments. The thermal treatments promote the formation of mechanical stress that cannot be compensated by a material having high mechanical strength.
Last but non least factor causing the problem of adhesion is the chemical action of the ink with respect to the photopolymer with time. The chemical resistance of a photopolymer to inks is of main importance because the material permeability to a liquid sooner or later will originate detachments at the photopolymer/substrate interface; said detachments, which can be followed by the liquid infiltration, generate electrical defects and, therefore, the malfunctioning of print heads.
U.S. Pat. No. 5,150,132 discloses an ink resistant material useful to make any component, particularly a top plate, of a print head having a surface contacting the ink. The material is disclosed to have high glass transition point and excellent heat resistant properties. The component of the print head is disclosed to be realised by moulding, preferably by casting moulding, compression moulding or compression moulding.
However, the Applicant has noticed that when the material according to the above patent is employed to make a photopolymer layer, the high glass transition point, particularly higher than 180° C., gives to the material an excessive mechanical strength favouring the detachment of the layer from metal surfaces of gold or other metals showing low adhesion characteristics. Further, the use of moulding techniques to realise the barrier or structural layer of a print head is difficult and expensive in view of the reduced dimensions, in the order of some micrometers, connected with the manufacturing thereof.
U.S. Pat. Nos. 6,455,112 and 6,638,439 disclose the use of polyfunctional epoxy resins having oxycyclohexane skeleton to form structural layers of an ink-jet print head.
However, the Applicant has noticed that the high mechanical strength and Tg of the above mentioned resin compositions promote the detachment of the photopolymer layer from metal surfaces of gold or other metals showing low adhesion characteristics.
U.S. Pat. No. 6,793,326 discloses that structural layers made of cationically polymerised product of alicyclic epoxy resins showed a peeling in case of high internal stress because of a high mechanical strength. The proposed solution suggests to make the structural layer by curing a radiation curable resin composition comprising an epoxy resin having at least two epoxy groups and obtained from the polymerisation of an acrylic monomers bearing epoxy groups.
However, the Applicant has noticed that the epoxy resin obtained from the polymerisation of an acrylic monomers bearing epoxy groups did not show the optimal adhesion characteristics required to form a photopolymer layer of an ink-jet print head.
U.S. Pat. No. 4,685,968 discloses an aqueous-based ink for ink-jet printers comprising a vehicle of about 5 to 95% by weight water and the balance at least one glycol ether, a dye, present in an amount up to about 10% by weight of the vehicle composition, and a cationic compound selected from the group consisting of alkanol ammonium compounds and cationic amide compounds, present in an amount such that there is at least one molecule of cationic compound for at least one of the negatively charged functional groups on the dye. Said ink compositions evidence minimal crusting and reduced evaporation loss of water thanks to the fact that the solubility of anionic dyes is increased in acidic media over that otherwise obtainable, due to the presence of the alkanol ammonium compound or amide cation; chloride is disclosed to be an appropriate anion to employ with the alkanol ammonium species, and hydrochloric acid is suitably used to reduce the pH.
However, the Applicant has noticed that the use of chloride anions cause corrosion effects on the print head.
European Patent No. 1 254 921 B1 discloses an ink-jet ink composition for ink-jet printers comprising at least one colorant and a vehicle including at least one organo-phosphonic acid, said composition being able to control bleed and yet to exhibit favourable interactions with the components of the pen structure and in particular with the adhesives commonly used in ink-jet printer pens, thus reducing structural adhesive swell.
However, the Applicant has noticed that the interactions between the ink-jet inks according to the above patent and print head structure still affect in particular the long term reliability of print head performance, being adhesion with time not sufficient.
The foregoing status of the art thus indicates that there is a need for a continuous research and improvement for ink-jet print heads that, when contacted by a water-based ink, are able to improve the adhesion with time, and can be easily employed in manufacturing processes.